Polyolefins, particularly polyethylene, polypropylene and .alpha.-olefin copolymers of ethylene and propylene, have heretofore been produced by a variety of processes ranging from solvent, slurry and gas phase polymerization processes which are carried out over wide ranges of temperature and pressure. The polymerization reaction of such processes have been conducted with a wide range of catalyst compositions ranging from conventional Ziegler-Natta type catalyst systems to alumoxane activated metallocene catalyst systems as described in European Patent No. 0,129,368 to the more recently disclosed ionic catalyst systems as described in European Patent Applications 0 277 004 and 0 277 003.
In solvent processes, polymerization of monomers occurs in the medium of a solvent, typically an inert hydrocarbon such as hexane, heptane, toluene or the like, which carries the catalyst into contact with monomer dissolved therein and typically the medium is one in which the product polymer is soluble. The solvent medium absorbs the heat generated by the polymerization reaction and control of the solvent medium temperature controls the temperature of reaction whereby optimum productivity or polymer properties may be achieved according to the characteristic of the catalyst used. After polymer production, the solvent medium and dissolved polymer must be separated, by a subsequent processing step, as by evaporation.
In slurry processes, monomer polymerization occurs in the medium of a fluid in which the polymer product is insoluble or poorly soluble and, as the polymer is produced, it precipitates or beads up in the medium while unreacted monomer remains in fluid form. The temperature of reaction is controlled by controlling the temperature of the slurry medium. The medium must be separated from the polymer product by a subsequent processing step. In those situations wherein the slurry medium is an inert normally liquid hydrocarbon compound distinct from the monomer itself, subsequent separation from the polymer product is accomplished by evaporation or filtration. When the medium for slurry. polymerization is the monomer itself produced by subjugation of the monomer to high pressures to convert it to a fluid form, separation of unreacted monomer medium from the polymer product is typically accomplished by causing the fluidized monomer to vaporize, or flash off, from the non-volatile polymer product. The unreacted monomer may be caused to flash off by significantly reducing its pressure or by adding additional heat to the medium, or both. Generally, because slurry polymerization processes are carried out at a temperature in the monomer reaction medium of less than about 80.degree. C., flashing of the unreacted monomer medium from the polymer product is accomplished by addition thereto of heat rather than by significant reduction of pressure as this would require significant costly recompression of recovered monomer before recycle to the reactor.
Whether a solvent or slurry procedure is used, ultimately the produced polymer must be separated from the polymerization medium which is generally accomplished by addition thereto of extra heat, which adds to the cost of polymer production.
The need to separate the solvent or slurry medium is a disadvantage in terms of the subsequent processing required. However, a solvent or slurry medium method of polymer production does enable one to control the temperature of the polymerization reaction to achieve the set of chemical and physical properties desired in the polymer product as those product characteristics are dictated by the nature of the polymerization catalyst system which is used.
The physical and chemical properties obtained in the polymer product, i.e., of molecular weight, molecular weight distribution, comonomer content and distribution, tacticity, etc., are significantly influenced by the type of catalyst system utilized, which in turn often dictates the nature of the polymerization process employed. Conventional Ziegler-Natta type catalysts, which comprise a Group IV-B metal compound and a metal alkyl cocatalyst such as an aluminum trialkyl, are highly active multi-sited catalysts which generally produce polymer products of a high molecular weight and broad molecular weight distribution. On the other hand, an alumoxane activated metallocene catalyst is a single sited catalyst system which generally produces a polymer of a narrow molecular weight distribution which may be of a relatively high molecular weight, particularly wherein the metallocene component is one of a Group IVB transition metal, particularly titanium or zirconium. However, to obtain a useful level of productivity with an alumoxane activated metallocene system when utilized in a solvent or slurry polymerization process, it is generally necessary to employ the alumoxane component in an amount such that the catalyst system has an aluminum atom to transition metal atom ratio of at least 1000:1, and typically much greater--i.e., 10,000:1 or greater. Lesser ratios, such as 12:1 to 100:1, can be employed such as that described in U.S. Pat. No. 4,752,597.
Although an alumoxane activated metallocene catalyst system has a variety of advantages relative to a conventional Ziegler-Natta type catalyst system, to be sufficiently active, such catalyst systems require the presence of a quantity of alumoxane which is undesirable in terms of the catalyst cost and the catalyst residue imparted into the polymer product produced therewith. As a consequence, a catalyst system has been developed wherein a transition metal component is activated to a catalytic state by reaction with certain types of ion exchange compositions, as described in commonly owned copending U.S. patent application Ser. Nos. 542,236 U.S. Pat. Nos. 5,198,401 and 5,278,119 and since described in European Patent Applications 0 277 003 and 0 277 004. Such ionic catalyst system are single-sited catalyst systems which produce polymers of a narrow molecular weight distribution at high levels of productivity wherein the ratio of ionic activator component to transition metal component is 1:1 or less. The transition metal component of such catalyst systems--like those in an alumoxane activated system--contains at least one ligand in the nature of a pi-bonding moiety, eg., a cyclopentadienyl group, hence may be referred to as a metallocene type catalyst system. In comparison to a conventional Ziegler-Natta catalyst the ionically activated metallocene catalyst system provides the same advantages of an alumoxane activated metallocene catalyst system while overcoming one of the aspects of an alumoxane activated system which was undesirable, namely the use of an excessive amount of costly alumoxane cocatalyst which also imparts a high content of catalyst residue to the polymer produced with a metallocene catalyst system.
For many applications it is of primary importance for a polyolefin to have a relatively high weight average molecular weight while having a relatively narrow molecular weight distribution. A high weight average molecular weight, accompanied by a narrow molecular weight distribution, provides a polyolefin or an ethylene-.alpha.-olefin copolymer with high strength properties. Generally, a desire for a polyolefin of this combination of properties dictates the use of a single sited metallocene-type catalyst system.
When an alumoxane activated metallocene catalyst system is employed, it has been found that a zirconium metallocene species is commonly more active than a hafnium or titanium analog for the polymerization of ethylene alone or together with another .alpha.-olefin to produce a copolymer. When employed in a non-supported form--i.e., as a homogenous or soluble catalyst system--in an alumoxane activated system it has been found that only the zirconium or hafnium species of a metallocene may be used wherein the reactor pressure exceeds about 500 bar (50 MPa; 7,252 psi) and reaction temperature exceeds 100.degree. C. (212.degree. F.). At such pressures and temperatures, the titanium species of metallocenes as activated by an alumoxane are generally unstable unless they are deposited upon a catalyst support.
Typically, the productivity of an alumoxane activated catalyst system is significantly greater in a solvent or slurry phase polymerization procedure than is the productivity of the same metallocene-alumoxane catalyst system when utilized in a high temperature and high pressure polymerization process.
For many reasons it is desirable to produce a polymer by a procedure wherein the temperature of the medium within which monomer polymerization occurs is as high as possible--i.e., a polymerization temperature greater than that of melting point and approaching the decomposition temperature of the product polymer. One reason for this desire is that increasing temperature should increase the rate of polymerization, which in turn would increase the rate of polymer production within a given unit of time. This would increase the capacity of a given reactor system for the production of polymer product. Another reason, particularly when a polymerization diluent is used as the medium in which polymerization occurs, is the simplification of treatment following polymer production to separate and recover the medium from the polymer product. In this case after polymer production the medium would comprise the polymerization diluent and unreacted monomers which may be separated from the polymer product by allowing the medium to flash off, or vaporize away from the non-volatile polymer, for recovery as a vapor to be condensed for reuse by recycle back to reactor. If polymerization could practically be accomplished while the medium is already at or in excess of its flash point temperature, then the medium would not need to be heated after removal from the reactor in order to separate and recover it from the polymer product. Since the heat of reaction of the polymerization reaction could be utilized as the source for heating the polymerization medium, the cost of extrinsically heating to subsequently flash the medium from the polymer product could be saved, again decreasing product cost.
Even when a diluent is not used and the polymerization medium is comprised of one or more monomers maintained in a fluid state by application of high pressures, it would still be desirable to conduct the polymerization reaction at a high temperature to increase the rate of polymer production. Further, wherein the product is a copolymer one monomer of which is of low volatility, i.e., a monomer of from C.sub.4 to C.sub.20 a , higher temperature for the polymerization medium would allow unreacted low volatility monomer to be flashed away from the polymer product with a slight pressure reduction and no or little additional heat input to the medium following its removal from the reactor.
To realize the desirable benefits which could stem from a high temperature of the polymerization medium requires the development of a catalyst system which is not adversely affected in its performance with respect to polymer productivity or polymer properties by a high temperature of the polymerization medium.
Heretofore, U.S. Pat. No. 5,084,534 has described the use of an alumoxane activated metallocene catalyst system for use in a high pressure--high temperature process for production of narrow molecular weight distribution polyolefin products. Unlike a relatively low temperature solution or slurry polymerization process wherein to achieve greater levels of productivity required increasing quantities of alumoxane to metallocene, in U.S. Pat. No. 5,084,534 it was found that under high temperature--high pressure polymerization conditions (i.e., at least 120.degree. C.; 248.degree. F.--500 bar; 50 MPa; 7,252 psi) the maximum level of catalyst productivity was instead achieved by limiting the quantity of alumoxane to an amount no greater than to provide the catalyst system with an Al:transition metal atom ratio of 1000:1 or less. By such limitation, when used in a high pressure-high temperature process the metallocene-alumoxane catalyst system is stated to have a high productivity--defined as 1000 g polymer/g catalyst or greater--the highest productivity exemplified being 4800 g polymer/g catalyst.
That level of catalyst productivity that may be achieved as a maximum in a high pressure-high temperature process as described by U.S. Pat. No. 5,084,534, is achieved at a temperature below that which is most desirable for process optimization in terms of medium-polymer separation, unreacted monomer recovery and reuse recycle operation. It has been found that in a high pressure polymerization process practiced in accordance with U.S. Pat. No. 5,084,534, that the metallocene-alumoxane catalyst productivity increases with temperature up to a range of about 140.degree. C. to about 160.degree. C. and thereafter declines significantly and rapidly with further increases of temperature of the polymerization medium. Accordingly, to maintain the reaction conditions at the state most favorable to maximum polymer productivity by the metallocene-alumoxane catalyst, the polymerization medium must be maintained at a controlled temperature by limiting catalyst concentration or by heat exchange so that the medium does not exceed a temperature of about 140.degree. C. to about 160.degree. C. Further, at this temperature, to flash unreacted monomer away from the polymer product without significantly reducing its pressure requires additional heat input to the medium after its removal from the reaction zone. The need to keep the medium at or below about 160.degree. C. during the polymerization reaction and thereafter to additionally heat it to flash and recover unreacted monomer from the product polymer without significant pressure reduction adds significantly to the cost of polymer production.
Though the benefits of polymerization at high temperatures--approaching that of the decomposition point of the product polymer--are apparent, to date no catalyst system has been found to be of practical use at such high temperatures wherein the desired product is a polyolefin of narrow molecular weight distribution and relatively high molecular weight. A need still exists for a polymerization process capable of attaining a high temperature in the polymerization medium which retains the several advantages heretofore achieved with single sited metallocene catalyst systems while enabling the efficient and economically attractive production of high molecular weight polymer products at high levels of productivity based upon the amount of catalyst employed.